It has previously been postulated that when plants are stressed by certain changes in patterns of weather they become a better source of food for invertebrate herbivores because this stress causes an increase in the amount of nitrogen available in their tissues for young herbivores feeding on them. And this may cause outbreaks of such phytophagous invertebrates. Evidence is now presented that a similar physiological mechanism appears to operate when a wide variety of apparently unrelated environmental factors impinge on plants or parts of plants in such a way as to perturb their metabolism. A broken branch, lightning strike, fire, nutrient deficiencies or an otherwise adverse site; all may have this effect. With the advent of modern man the available agencies increase and diversify to include pesticides, irradiation and air pollutants. One common metabolic response by plants to all such agents impinging on them seems to be equivalent to that found in senescing plant tissues — the breakdown and mobilization of nitrogen in soluble form away from the senescing/stressed tissues. Young herbivores which chance to feed on such stressed/senescing tissues have a greater and more readily available supply of nitrogen in their food than they would have had if feeding on unstressed plants. As a result many more of them survive, and there is an increase in abundance of their kind. Such increases may be quite localised and short-lived or more widespread and persistent, depending on the extent and duration of the stress experienced by the plants. And in the face of this improved nutrition and survival of the very young, predators and parasites seem to have only a minor influence on subsequent changes in abundance of their herbivorous prey. Another effect of increased mobilization of nitrogen in stressed plants is an increase in the quantity of the seed that they set. This has led to the conclusion that increased abundance of some species of birds at such times is due to a greater supply of seeds as winter food for recent fledglings. But it may be that the increased abundance is due to the synchronous increase in phytophagous insects providing a richer source of protein food for laying hens and growing nestlings.

https://www.jstor.org/stable/info/4217356

The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants.

Behavioural aspects influencing plant virus transmission by homopteran insects.

Pathogens and parasites can induce changes in host or vector behavior that enhance their transmission. In plant systems, such effects are largely restricted to vectors, because they are mobile and may exhibit preferences dependent upon plant host infection status. Here we report the first evidence that acquisition of a plant virus directly alters host selection behavior by its insect vector. We show that the aphid Rhopalosiphum padi, after acquiring Barley yellow dwarf virus (BYDV) during in vitro feeding, prefers noninfected wheat plants, while noninfective aphids also fed in vitro prefer BYDV-infected plants. This behavioral change should promote pathogen spread since noninfective vector preference for infected plants will promote acquisition, while infective vector preference for noninfected hosts will promote transmission. We propose the “Vector Manipulation Hypothesis” to explain the evolution of strategies in plant pathogens to enhance their spread to new hosts. Our findings have implications for disease and vector management.

/pdf/plant-viruses-alter-insect-behavior-to-enhance-their-spread-32t1dt8yqx.pdf

Plant viruses alter insect behavior to enhance their spread

The influence of viral disease symptoms on the behaviour of virus vectors has implications for disease epidemiology. Here we show that previously reported preferential colonization of potatoes infected by potato leafroll virus (genus Polerovirus) (luteovirus) (PLRV) by alatae of Myzus persicae, the principal aphid vector of PLRV, is influenced by volatile emissions from PLRV-infected plants. First, in our bioassays both differential immigration and emigration were involved in preferential colonization by aphids of PLRV-infected plants. Second, M. persicae apterae aggregated preferentially, on screening above leaflets of PLRV-infected potatoes as compared with leaflets from uninfected plants, or from plants infected with potato virus X (PVX) or potato virus Y (PVY). Third, the aphids aggregated preferentially on screening over leaflet models treated with volatiles collected from PLRV-infected plants as compared with those collected from uninfected plants. The specific cues eliciting the aphid responses were not determined, but differences between headspace volatiles of infected and uninfected plants suggest possible ones.

http://europepmc.org/articles/PMC1690919?pdf=render

Volatiles from potato plants infected with potato leafroll virus attract and arrest the virus vector, Myzus persicae (Homoptera: Aphididae)

The factors responsible for determining the host-plants and feeding sites of aphids, and the various probing activities (the role of the labium, stylet insertion, surface saliva deposition, the behaviour of the aphid, virus transmission) are examined. There is a brief review of stylet structure and movement and the possible sensory nature of these organs, followed by a detailed review of the characteristics of aphid stylet paths in plant tissues. The penetration of epidermis and vascular tissues is treated separately while that within the intermediate tissues is covered in relation to leaves and stems, roots, trees, galls and excised tissue as well as in separate sections on Aphis fabaeScopoli and Myzus persicae (Sulzer). Stylet destinations and behaviour in the sieve tubes are discussed together with general features such as rate and depth of penetration, guidance to the feeding site, effects of tissue hardness and stylet withdrawal. The ingestion rate of plant sap is reviewed and its constitution and importance examined together with the significance of artificial diets. The salivary secretions including sheaths and tracks, their functions and their role in the transference of material between aphid and host are dealt with. The nature of the physical and internal damage resulting from aphid feeding is briefly covered, and also some plant-insect interrelations. The aphid species whose stylets have been examined in plant tissue are listed.

Plant penetration by feeding aphids (Hemiptera, Aphidoidea): a review

IT has been shown that robust green leaves which have ceased growth and not yet entered upon senescence are unsuitable, by comparison with growing or senescing leaves, for colonization by Aphis fabae Scopoli and Myzus persicae (Sulzer)1–3. Abnormally extended growth or premature senescence-like changes, or both, are characteristic of both the ‘pseudo-galls’ (curling and pocketing of leaves) and the virus diseases which plants can develop as a result of aphid attack. Experiments were therefore undertaken to see whether such pathological changes in the plants might not be beneficial to the aphids initiating them.

J. S. Kennedy

Papers

Benefits to aphids from feeding on galled and virus-infected leaves.